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• Boost Python : How to only expose the constructor of a class with virtual (pure & impure) methods

  - by fallino

  Hello, I'm a newbie with Boost::Python but I tried to search on the web to do so I want to expose a 3rd party library to Python. One of the class of the library (.hpp) is composed of a public constructor with arguments a protected constructor and functions various regular functions various pure virtual functions various non pure virtual functions First, I did not succeed in building it without having errors about this protected constructor. I finally commented it. A first question would be : Is there a way to exclude these protected functions since I don't want to expose them ? (I know it's possible and easy with Py++, but I started without using it) Then I tried to expose all of my functions, beginning with the pure virtual ones (commenting them all except one), which wasn't a success too So I finally decided not to expose these virtual functions (which in fact seems logical...), but, here again, I didn't manage building it with a simple constructor with arguments (without no_init). So my second question is : Is there a way to exclude these virtual functions since I don't want to expose them ? Sorry if it seems trivial but I didn't find anything explicit on the web and I need something rather explicit :). Thanks in advance

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• How to use pure in D 2.0

  - by James Dean

  While playing around with D 2.0 I found the following problem: Example 1: pure string[] run1() { string[] msg; msg ~= "Test"; msg ~= "this."; return msg; } This compiles and works as expected. When I try to wrap the string array in a class I find I can not get this to work: class TestPure { string[] msg; void addMsg( string s ) { msg ~= s; } }; pure TestPure run2() { TestPure t = new TestPure(); t.addMsg("Test"); t.addMsg("this."); return t; } This code will not compile because the addMsg function is impure. I can not make that function pure since it alters the TestPure object. Am i missing something? Or is this a limitation? The following does compile: pure TestPure run3() { TestPure t = new TestPure(); t.msg ~= "Test"; t.msg ~= "this."; return t; } Would the ~= operator not been implemented as a impure function of the msg array? How come the compiler does not complain about that in the run1 function?

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• How does a "Variables introduce state"?

  - by kunj2aan

  I was reading the "C++ Coding Standards" and this line was there: Variables introduce state, and you should have to deal with as little state as possible, with lifetimes as short as possible. Doesn't anything that mutates eventually manipulate state? What does "you should have to deal with little state as possible" mean? In an impure language such as C++, isn't state management really what you are doing? And what are other ways to "deal with as little state as possible" other than limiting variable lifetime?

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• Manual memory allocation and purity

  - by Eonil

  Language like Haskell have concept of purity. In pure function, I can't mutate any state globally. Anyway Haskell fully abstracts memory management, so memory allocation is not a problem here. But if languages can handle memory directly like C++, it's very ambiguous to me. In these languages, memory allocation makes visible mutation. But if I treat making new object as impure action, actually, almost nothing can be pure. So purity concept becomes almost useless. How should I handle purity in languages have memory as visible global object?

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• In what areas might the use of F# be more appropriate than C#?

  - by Peter McGrattan

  Over the last few years F# has evolved into one of Microsoft's fully supported languages employing many ideas incubated in OCaml, ML and Haskell. Over the last several years C# has extended it's general purpose features by introducing more and more functional language features: LINQ (list comprehension), Lamdas, Closures, Anonymous Delegates and more... Given C#'s adoption of these functional features and F#'s taxonomy as an impure functional language (it allows YOU to access framework libraries or change shared state when a function is called if you want to) there is a strong similarity between the two languages although each has it's own polar opposite primary emphasis. I'm interested in any successful models employing these two languages in your production polyglot programs and also the areas within production software (web apps, client apps, server apps) you have written in F# in the past year or so that you would previously have written in C#. EDIT: Edited based on feedback from close votes with the intent of reducing perceived ambiguity.

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• Why darcs instead of git?

  - by Ctrl Alt D-1337

  Using pure functional languages can have a lot of benefits over using impure imperatives but low level systems languages will generally allow you to achieve much greater performance especially when they are imperative because it allows you to specify the exact steps in how the cpu should compute the result. If there is ever list of tools where high performance is an absolute must then I would put source version controls systems right at the top of that list and git achieves this very well but performance is not it's only advantage over many other other types of version control systems anyway. The git team are handling the unsafe c code very well and I never worry about my type system or any other features of the language it is written in so why is it that there is a lot of haskell developers that must use darcs when they will only be using the finished product?

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• What production software have you written in F# in the past year or so that you would previously hav

  - by Peter McGrattan

  Over the last few years F# has evolved into one of Microsoft's fully supported languages employing many ideas incubated in OCaml, ML and Haskell. Over the last several years C# has extended it's general purpose features by introducing more and more functional language features: LINQ (list comprehension), Lamdas, Closures, Anonymous Delegates and more... Given C#'s adoption of these functional features and F#'s taxonomy as an impure functional language (it allows YOU to access framework libraries or change shared state when a function is called if you want to) there is a strong similarity between the two languages although each has it's own polar opposite primary emphasis. I'm interested in any successful models employing these two languages in your production polyglot programs and also the areas within production software (web apps, client apps, server apps) you have written in F# in the past year or so that you would previously have written in C#.

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• In what specific areas has F# proven more applicable than C#?

  - by Peter McGrattan

  Over the last few years F# has evolved into one of Microsoft's fully supported languages employing many ideas incubated in OCaml, ML and Haskell. Over the last several years C# has extended it's general purpose features by introducing more and more functional language features: LINQ (list comprehension), Lamdas, Closures, Anonymous Delegates and more... Given C#'s adoption of these functional features and F#'s taxonomy as an impure functional language (it allows YOU to access framework libraries or change shared state when a function is called if you want to) there is a strong similarity between the two languages although each has it's own polar opposite primary emphasis. I'm interested in any successful models employing these two languages in your production polyglot programs and also the areas within production software (web apps, client apps, server apps) you have written in F# in the past year or so that you would previously have written in C#. EDIT: Altered title with the intent of reducing perceived ambiguity.

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• Database Functional Programming in Clojure

  - by Ralph

  "It is tempting, if the only tool you have is a hammer, to treat everything as if it were a nail." - Abraham Maslow I need to write a tool to dump a large hierarchical (SQL) database to XML. The hierarchy consists of a Person table with subsidiary Address, Phone, etc. tables. I have to dump thousands of rows, so I would like to do so incrementally and not keep the whole XML file in memory. I would like to isolate non-pure function code to a small portion of the application. I am thinking that this might be a good opportunity to explore FP and concurrency in Clojure. I can also show the benefits of immutable data and multi-core utilization to my skeptical co-workers. I'm not sure how the overall architecture of the application should be. I am thinking that I can use an impure function to retrieve the database rows and return a lazy sequence that can then be processed by a pure function that returns an XML fragment. For each Person row, I can create a Future and have several processed in parallel (the output order does not matter). As each Person is processed, the task will retrieve the appropriate rows from the Address, Phone, etc. tables and generate the nested XML. I can use a a generic function to process most of the tables, relying on database meta-data to get the column information, with special functions for the few tables that need custom processing. These functions could be listed in a map(table name -> function). Am I going about this in the right way? I can easily fall back to doing it in OO using Java, but that would be no fun. BTW, are there any good books on FP patterns or architecture? I have several good books on Clojure, Scala, and F#, but although each covers the language well, none look at the "big picture" of function programming design.

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